Thermal treatment of aluminum base alloy product



United States Patent Pennsylvania No Drawing. Filed Nov. 16, 1962, Ser. No. 238,263

5 Claims. (Cl. 148-115) This invention relates to the thermal treatment of aluminum-manganese type alloys in different stages in the production of various wrought articles, and it is more particularly concerned with obtaining a fine uniform grain size in the final annealed product. Alloys of this type are not subjected to solution heat treatment and subsequent precipitation hardening in commercial operations to increase their strength and hardness, but are simply work hardened to gain strength in addition to that provided by the presence of manganese. For this reason, they are frequently referred to as being non-heat treatable alloys. In the course of making semi-finished as well as finished products of these alloys, it is a common practice to relieve the strains generated by the working operation by an annealing treatment. Where the annealing is employed between metal working steps it is called intermediate annealing and where the finished product is softened the treatment is referred to as being a final anneal.

Annealing or softening of work hardened aluminummanganese alloys is usually accomplished by heating the alloys to a temperature between about 650 and 800 F. and cooling to room temperature at an uncontrolled rate. Usually a short holding or soaking period of 1 to 2 hours is allowed at the predetermined temperature to insure relief of work hardening strains and the recrystallization that accompanies such relief.

While the grain size of the annealed products is satis factory for many purposes, it often becomes critical where the product has received a final anneal and is then subjected to a small reduction in cross section or is subjected to a local deformation as in a forming or shaping operation. If the annealed product exhibits large or coarse grains the subsequent general or localized working may produce what is known as the orange peel defect. This defect is characterized by a surface having a nonuniform rippled appearance and is visible to the naked eye. Although grain growth may be controlled to some extent by the addition of very small amounts of certain high melting point elements, such additions not only add to the cost of the alloy but introduce problems in the melting, casting and working operations and do not insure a fine grain size in the finished product. We have found that a fine uniform grain size can be produced without the addition of any high melting point grain refining elements by employing a novel combination of working and thermal treatments.

It is an object of our invention to provide a method of thermally treating aluminum-manganese type alloys in the course of making wrought products therefrom which will yield a uniform fine grain size when the products are annealed, especially at the stage of a final anneal. A particular object is to provide a series of thermal treatments which will substantially decrease or prevent the occurrence of the orange peel defect.

Our invention is predicated upon the discovery that by initiating precipitation of the manganese constituent in an aluminum-manganese type of alloy, dissolving at least a portion of the precipitated manganese in the worked product, subsequently causing a second precipitation of a substantial part of the dissolved manganese in the proper size and distribution and cold working that product at least 10%, the growth of large or coarse grains is inhibited in that product when annealed. Manganese is difiicultly soluble in solid aluminum even though the solubility increases with temperature. The maximum amount of manganese which can be dissolved in solid aluminum under equilibrium conditions in a binary alloy, is about 1.8% at a temperature of 1215 to 1220 F. but obviously this amount is not dissolved under commercial operating conditions. For the purpose of our treatment, however, it is not necessary that the maximum amount be dissolved but rather that only a substantial portion of the manganese which can be dissolved at a given temperature should .be placed in solution. It will be appreciated that the mass of alloy being heated, the maximum temperature attained, the amount of manganese which is present and possibly other elements will afiect the quantity of manganese which will be dissolved.

The alloys which are benefited by our treatment contain from 0.5 to 2.0% manganese and the usual impurities of up to 0.75% iron, 0.4% silicon, up to 0.1% zinc and up to a total of 0.15% of others of a minor character. To increase the strength of the alloy it may be desirable to add from 0.1 to 2 .0% magnesium but where this is done, the silicon content must be limited to less than 0.2%. Copper occurs as an impurity in the alloy, usually less than 0.1%, but to control action of the alloy under corrosive conditions it may be desirable to increase the copper content to 0.35%. The presence of magnesium and copper do not disturb the response of the manganese to the thermal treatment.

In making wrought products it is, of course, common to start with an ingot or other suitably cast form adapted to being worked. Before working the cast body, it should be heated to a temperature between 1050 and 1150 and held within that temperature range for 4 to 72 hours. This treatment, referred to herein as a preheat treatment, aids in developing a more uniform structure in the cast body, minimizes any efiects of segregation and initiates at least some precipitation of the manganese-containing constituent. The heated body is cooled to the hot working temperature range and hot worked, or it can be further cooled to room temperature and then be reheated to the working temperature. To secure the best results the cooling from the preheating to the hot working temperature range should be at a slow rate, less than 50 F. per hour. The cooling in any case should not be drastic such as by quenching. Although it is a general practice to hot work a cast body to obtain a relatively large reduction in thickness, in some cases it may be possible to cold work the thermally treated ingot, slab or bar. In any case the cast structure is broken down by the Working thereby producing a wrought structure. Any hot Working is generally performed within a temperature range of 650 to 900 F. and the worked product cooled to a lower temperature (below 650 F.), and preferably to room temperature for the sake of convenience in handling.

While it is generally preferred to preheat and hot work an ingot, as described above, it is not essential that an ingot be the body which is preheated. An intermediate product such as a slab, bar or rough forging made from an ingot by following conventional heating and working practices, can be subjected to the first step in our process, namely a high temperature preheat at 1050 to 1150 F., and then hot worked as in the case of Working an ingot. The ingot or intermediate worked product is herein re ferred to as a body.

Contrary to the behavior of many elements which are soluble in solid aluminum, the maximum solution of manganese appears to exist in the ingot in the as-cast condition and subsequent preheating and hot working induces some precipitation. Precipitation of this character seems to be essential to establishing the proper condition for subsequent steps and attainment of a fine grain size in the final product.

The product of the foregoing heating and working operations is next reheated to a temperature between 850 and 1025 F. and held within that temperature range for a period of 1 to 30 hours. Generally, a longer time at temperature is desirable where the soaking temperature is in the lower portion of the temperature range than where a higher temperature treatment is employed. In any case the heating must continue for a sufficient length of time to permit the solution of at least a portion of the manganese, within its limits of solubility, that was precipitated during the preceding heating and working operations.

Upon completion of the foregoing solution treatment the heated body of the alloy should be cooled to at least 700 F., at a controlled rate of 15 to 50 F. per hour whereby precipitation of a manganese-containing alloy constitutent is produced. The cooling rate below 700 F. need not be controlled, and hence the treated product can be allowed to cool to room temperature without further control. The precipitate should be finely divided and uniformly distributed in order to control the grain size of the final product. The cooling rate is critical within the temperature range between 700 F. and the solution temperature, rates of less than 15 F. per hour are too slow, there being insufiicient additional benefit to justify the increased length of time, while rates in excess of 50 F. per hour fail to consistently yield a fine grain size in the final annealed product. The range of rates must therefore be carefully observed to achieve the desired precipitation.

The product of the precipitation treatment is cold worked with a reduction of at least 10% in cross section. Smaller reductions tend to promote the growth of coarse grains when the cold worked article is subsequently annealed.

Although the solution treatment may be employed at any intermediate stage in the fabrication of wrought products, the best results are generally obtained if it and the precipitation treatment immediately precede the final cold working operation.

The final step in the series of treatments is that of annealing the cold worked product to remove work hardening strains. Normally this consists of heating the product to a temperature between 650 and 800 F., with or without a short period of holding within that temperature range and cooling to room temperature. Under these conditions no solution occurs of the precipitated manganese constituent and thus adversely affect the grain refining effect of the constituent in the annealed product. The annealed product will have a grain size of more than about 2500 grains per cubic millimeter, and even grain sizes of 10,000 grains or more per cubic millimeter can be attained. This is in contrast to the coarse grains generally obtained heretofore by the usual working and annealing practices where the grain count is on the order of less than 1000 grains per cubic millimeter.

The following examples illustrate the benefits gained from the treatment.

An alloy consisting of 1.10% manganese, 0.15% copper, 0.02% titanium, 0.65% iron, 0.25% silicon and balance aluminum was cast by the continuous casting process in the form of an ingot having a cross section 6" x 16". Two transverse sections, designated A and B, were cut from the ingot and were given different preheating treatments prior to being hot rolled to plate thickness. Section A was heated to 1 l F. and held at that temperature for 16 hours, then cooled in air to 900 F. and hot rolled to 0.3" thick plate. Section B was given a conventional preliminary treatment consisting of heating at 900 F. for 16 hours and then hot rolled to 0.3" thick plate.

In each case the hot rolled plate was cooled to room temperature before proceeding to the next step in the process.

Three pieces were cut from each of the hot rolled plates for further treatment and cold rolling. These pieces were heated to 950 F. and held at that temperature for 1 hour following which one piece from each ingot section was cooled to 700 F. at a rate of 15 F. per hour, a second piece was cooled at a rate of 30 F. per hour to 700 F. and the third piece was cooled at a rate of 45 F. per hour to 700 F. All pieces were cooled in air from 700 F. to room temeprature following the controlled cooling schedule and were then cold rolled to sheet 0.064" in thickness. The cold rolled sheet was given a slow batch anneal at 650 F. to determine the effect of the preceding treatments on the grain size of the annealed product. The annealed sheets were examined for grain size and count in accordance with the procedure described in the American Society of Testing Materials Standards, Volume 3, E11258T (1958) pages 506 to 520 to give the number of grains per cubic millimeter. The ingot sections, the rates of cooling of the hot rolled plates during the precipitation step, and the grain counts are given in Table I below.

TABLE I Grain count in annealed sheets Ingot section Cooling rate from 950 Grain count (gralns/ F. to 700 F. mmfi) The foreging test results demonstrate that the initial thermal treatment of the alloy body is important and that the preheating must be done at a higher temperature than used in conventional practice. The preheating at 900 F. was ineffective even though followed by the same solution and precipitation steps as applied to the plates from ingot section A. The coarse grain size in sheet made from the B section of the ingot would be cause for rejection, especially if any forming operations were to be performed. To obtain a fine grain size it is obvious that the proper initial treatment must be employed. Cooling should follow the solution treatment immediately preceding the cold working operation.

Having thus described our invention and certain embodiments thereof, we claim:

1. The method of producing a uniform fine grain size in annealed wrought products of an alloy consisting essentially of aluminum and 0.5 to 2.0% manganese, said method comprising providing a body of said alloy containing a substantial proportion of the manganese content in solution, preheating said body at 1050 to '1150 F. for a period of 4 to 72 hours to initiate precipitation of the dissolved manganese, cooling said preheated body to a hot working temperature range of 650 to 900 F., hot working said body and thereafter cooling below the minimum hot working temperature, reheating said worked body to between 850 and 1025 F. and holding within that temperature range for a period of 1 to 30 hours and for a sufiicient length of time to cause a re-solution of at least a portion of the previously precipitated manganese, slowly cooling the said solution heat treated product to 700 F. at a rate of 15 to 50 F. per hour, continuing the cooling to room temperature at an uncontrolled rate, cold working said cooled product with a reduction of at least 10% and finally annealing the cold worked product at 650 to 800 F.

2. The method according to claim 1 wherein the cooling from the preheating temperature to the hot working temperature is at a controlled rate of less than 50 F. per hour.

3. The method of claim 1 wherein the aluminummanganese alloy also contains 0.1 to 2% magnesium and the silicon impurity content is less than 0.2%

4. The method of claim 1 wherein the aluminum-manganese alloy also contains up to 0.35% copper.

5. The method of producing a uniform fine grain size in annealed wrought products consisting essentially of aluminum and 0.5 to 2.0% manganese, said method comprising casting a body of said alloy which contains a substantial proportion of the manganese content in solution, preheating said cast body at 1050 to 1150 F. for a period of 4 to 72 hours to initiate precipitation of the dissolved manganese, cooling to the hot Working temperature range, hot Working said preheated cast body at 650 to 900 F., cooling the worked body to below the minimum hot Working temperature, re-heating said Worked body to between 850 and 1025 F. and holding within that temperature range for a period of 1 to 30 hours and for a suflicient length of time to cause a ire-solution of at References Cited by the Examiner UNITED STATES PATENTS 2,966,731 1/1961 Towner et al. 75-138 OTHER REFERENCES Physical Metallurgy of Aluminum, published by the A.S.M., 1949, p. 209 relied on.

DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, Examiner. 

1. THE METHOD OF PRODUCING A UNIFORM FINE GRAIN SIZE IN ANNEALED WROUGHT PRODUCTS OF AN ALLOY CONSISTING ESSENTIALLY OF ALUMINUM AND 0.5 TO 2.0% MANGANESE, SAID METHOD COMPRISING PROVIDING A BODY OF SAID ALLOY CONTAINING A SUBSTANTIAL PROPORTION OF THE MANGANESE CONTENT IN SOLUTION, PREHEATING SAID BODY AT 1050 TO 1150* F. FOR A PERIOD OF 4 TO 72 HOURS TO INITITATE PRECIPITATION OF THE DISSOLVED MANGANESE, COOLING SAID PREHEATED BODY TO A HOT WORKING TEMPERATURE RANGE OF 650 TO 900*F., HOT WORKING SAID JBODY AND THEREAFTER COOLING BELOW THE MINIMUM HOT WORKING TEMPERATURE, REHEATING SAID WORKED BODY TO BETWEEN 850 AND 1025*F. AND HOLDING WITHIN THAT TEMPERATURE RANGE FOR A PERIOD OF 1 TO 30 HOURS AND FOR A SUFFICIENT LENGTH OF TIME TO CAUSE A RE-SOULTION OF AT LEAST A PORTION OF THE PREVIOUSLY PRECIPITATED MANGANESE, SLOWLY COOLING THE SAID SOLUTION HEAT TREATED PRODUCT TO 700* F. AT A RATE OF 15 TO 50*F. PER HOUR, CONTINUING THE COOLING TO ROOM TEMPERATURE AT AN UNCONTROLLED RATE, COLD WORKING SAID COOLED PRODUCT WITH A REDUCTION OF AT LEAST 10% AND FINALLY ANNEALING THE COLD WORKED PRODUCT AT 650 TO 800*F. 